A restrictor device controls a return flow of drilling fluid from a borehole where drilling fluid is provided from a surface installation via a drill string to a bottom-hole assembly. Drill pipes, forming the drill string, are provided with pipe joints including enlarged-diameter pipe portions. An annulus is formed between a pipe, for example a marine riser, and the drill string, and the annulus is in fluid communication with or forms parts of a drilling-fluid flow path. The restrictor device is arranged in the annulus, the restrictor device being formed of a restrictor pipe, anchored to said riser, and surrounding at least one restrictor element placed on an intermediate portion connecting two enlarged-diameter pipe portions. The length of the restrictor device exceeds the largest distance between two consecutive restrictor elements.
It is known to install drill-pipe/casing protectors (DPPs) externally on a drill string to prevent undue wear on the internal wall of the casing owing to the rotation of the drill string. The DPP, which is formed as cylindrical, dividable pads, typically formed from steel-reinforced rubber, is fitted to desired portions of the drill string as it is being built up, the two halves being locked to each other by means of locking pins.
From the applicant's own patent application PCT/NO2012/050187 a choke is known, arranged externally on a drill string, the length of the choke exceeding the distance between the enlarged joint portions of two consecutive drill pipes. The choke is placed in and attached to a riser. The purpose of the choke is to restrict a portion of the annulus in the riser above a return path for a drilling fluid in the form of a drilling-fluid return branch in the riser, where a drilling-fluid return line including a return-fluid pump is extended parallel to the riser to a surface installation. The annulus pressure below the choke, and thereby the bottom-hole pressure, may then be adjusted by controlling the capacity of the return-fluid pump, without any adjustment of the fluid column in the riser being necessary. The annulus pressure can thereby quickly be adjusted to the bottom-hole pressure requirement arising on a change between circulation and non-circulation in the drilling-fluid flow.
A problem with other prior art—in which the fluid column is reduced—is that when circulation ceases and, with that, there is a loss of frictional pressure in the borehole, the bottom-hole pressure could drop below the formation pressure so that well fluid starts to enter the borehole (well kick).
A drawback of the choke according to PCT/NO2012/050187 is that it represents a cer-tain risk of wear to the joint portions of the drill string because of the limited clearance, which is preferably in the range of 1.5-3 mm. Another drawback is that the diameter of the joint portions may vary somewhat, which gives variation in the choking effect and thereby variation in the leakage past the choke. The leakage will necessarily have to be overcome by the return-fluid pump and is thus lost energy and pump capacity, which could otherwise have been used for circulation through the borehole.
The applicant's own patent application PCT/NO2012/050187 is incorporated in its entirety herein by reference.